The present invention relates to installing cable suspensions and particularly cable stringing blocks used to install cables such as power transmission and distribution conductor cables, as well as communications cables.
There is a continuing need by power utility companies to string power conductor cables within their transmission and distribution networks. There are generally two types of power networks: transmission networks which carry very high voltage power cables over long distances from power plants to substations and distribution networks which carry lower voltage power cables to the power consumer.
Installation of transmission and distribution power conductor cables typically requires the use of a stringing block having a sheave wheel. Transmission stringing blocks generally use wheels having a diameter ranging from about 10 inches to about 50 inches, with 16 to 22 inch wheels being most common. Distribution stringing blocks generally use wheels having a diameter ranging from about 3 inches to about 10 inches, with a seven inch wheel being most common.
Current commercially available cable stringing blocks use aluminum wheels in an aluminum frame. These aluminum wheels tend to wear out quickly and damage the conductor cable being installed. Sherman and Reilly, Inc., Chattanooga, Tenn., and Bethea Power Products (a division of MacLean Power Systems), Pelham, Ala., are two known manufacturers of cable stringing blocks.
Recognizing the wear problems associated with conventional aluminum wheels, some manufacturers of cable stringing blocks have begun lining the aluminum wheel with conductive neoprene or polyurethane. The neoprene and polyurethane provide some protection to the conductor cable, but they wear out quickly, providing only temporary protection. In addition, the polyurethane and neoprene are susceptible to weather and ultraviolet rot. Most recently, one manufacturer has proposed replacing the aluminum wheels with cast iron as a solution to the foregoing problems.
Both the wheel and frame have historically been constructed of a conducting material for safety reasons, explained below. When installing power transmission cable, particularly where a new line is installed parallel to a live line, there is a serious potential of generating a static charge within the new line. Powerful electric charges have been known to jump from the new line to the ground. Recognizing this risk, it has been common to ground the new line through the stringing blocks. Even though the stringing blocks are made of conducting materials, applicants have observed that conventional stringing blocks are not sufficiently conductive to provide an adequate ground. They have been known to burn up when powerful electric charges surge through the blocks.
Another, lesser known, problem with conventional cable stringing blocks arises from their aluminum parts, i.e., the aluminum frame and wheel. Aluminum scavengers have been known to steal the stringing blocks from construction sites and sell them for their scrap aluminum value.
It will be appreciated that there remains a need in the art for cable stringing blocks which utilize wheels that are durable, yet protect the cable being strung. It also would be an advancement in the art to avoid the danger and problems associated with conductive wheels and blocks. It would be a significant improvement in the art to provide a way of reducing the theft of aluminum distribution blocks.
The present invention is directed to a cable stringing block assembly used to install cables such as power and communication cables. The block includes a frame structure, an axle assembly, and a non-conducting sheave wheel.
The frame is preferably constructed of cast aluminum. The frame can be anodized to disguise its aluminum appearance from scrap aluminum thieves and provide additional weather protection.
The wheel may vary in size depending upon the cable and required load. For example, when installing power distribution cable, the wheel size typically ranges from three to ten inches. When installing power transmission cable, the wheel size typically ranges from ten to fifty inches.
Importantly, the wheel is fabricated of a durable polymeric material. In a preferred embodiment the polymeric material is self-lubricating, having a low coefficient of friction less than 0.5. A variety of engineering plastics can be used, particularly those containing nylon. One currently preferred polymeric material is PA6-G-type nylon.
In a preferred embodiment, a handle is affixed to the frame structure to facilitate transport and positioning of the block assembly. A spring-biased fly gate is preferably provided to facilitate side-loading of the power conductor cable. Unlike other known spring-biased fly gates, the fly gate according to the present invention does not lock to the frame to provide structural support. A helicopter loading arm can be used in connection with the fly gate to facilitate helicopter loading of the cable.